Method for bonding porous coating to rigid structural member

ABSTRACT

A method for bonding a layer of porous polymeric material to a rigid implantable portion of a prosthetic device, which includes the steps of (a) forming a substrate by coating the implantable portion with material which is chemically compatible with the porous polymeric material and which has a melt viscosity and/or melting temperature lower than that of the porous polymeric material, (b) covering the substrate with a porous polymeric material in powder form capable of forming said porous polymeric layer for forming a composite coating on the implantable portion, and (c) heating the substrate and porous polymeric material to a temperature high enough to cause the substrate material to flow into the adjacent pores and low enough so that the porous material will maintain its shape so that the porous layer will be bonded to the substrate and the outer pores will be open to accommodate tissue ingrowth, and an apparatus formed by said method.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 764,952, filed Feb. 2, 1977, now abandoned.

This invention relates to a method for anchoring a thin layer of aporous polymeric material to the rigid structural component of asurgical implantation device and, more particularly, to a method whichincludes the step of forming a substrate of a material with a meltviscosity and/or melting temperature lower than that of the porousmaterial between the structural component and the porous outer layer.

Surgical implantation devices which utilize tissue ingrowth to anchorthe devices in place in the human body are being used more frequently.U.S. patent application No. 567,296, filed Apr. 11, 1975, and issued asU.S. Pat. No. 3,986,212, assigned to the same party as the instantinvention is directed to the use of porous polymeric materials as acoating on the outer surface of structural components of jointprostheses where a spike or stem portion is inserted into the medullarycanal and the teachings of that patent are hereby incorporated into thisapplication by reference. As described in the above patent, the porousmaterial is effective in accommodating and promoting tissue ingrowththroughout the entire porous layer, but strong enough when ingrowthoccurs to withstand stresses to which the devices are subjected.

Such coatings have conventionally been formed by a physical connectionbetween the porous layer and the underlying structural component. Forexample, the underlying unit has been modified by forming fenestrations,grooves, slots, undercut channels or other surface irregularities forholding the porous layer in place. It has been found, however, that thebond strength between the porous layer and the structural member isrelatively low because there are large areas of contact with no adhesionand the area of the porous layer which is actually bonded to thestructural member is small relative to the total contact surface. Theprovision of holes or grooves in the underlying structural member is nota satisfactory solution since they tend to weaken the prosthesis andcompensation must be made in the design by strengthening the member.Another disadvantage of a coating which is not adhered to the prosthesisalong the entire contact surface is that any exposed edges which are notdirectly adhered are susceptible to fracture, peel or breakage.

SUMMARY OF THE INVENTION

The problems discussed above are solved, in accordance with theinvention, by providing an interfacial layer or substrate between theunderlying structural component and the porous coating, which forms abond between the porous layer along its entire contact surface andprovides a remarkably greater "pull" or "peel" strength.

The underlying surface is roughened or otherwise prepared by providinggrooves or other irregularities thereon. A knurled surface formed by anumber of shallow grooves has been found to be preferable. It has beenfound that because of the significantly greater bond strength providedby the inventive process, the grooves need not be as deep as those usedpreviously so that the underlying structural member is not weakened.

The substrate is chemically compatible with the material of the porousouter layer and is coated on the underlying structural member in a thinlayer by a suitable technique such as, for example, dipping, brushing orflowing (e.g., where powder is sprinkled on and melted). For example,where porous polyethylene is used as the porous material, non-porouspolyethylene can be used for the substrate.

The porous layer is formed on the outer surface of the substrate. Thematerial used for the substrate has a melt viscosity and/or meltingtemperature lower than that of the porous material so that theinterfacial connection between the substrate and porous layer is formedby heating the composite which causes the substrate to flow into theadjacent pores. Although the surface preparation of the underlying unitis significantly less severe than for prior art devices where the porouslayer was applied directly to the underlying surface, the bond strengthis substantially greater. This is believed to be caused by the fact thatsince the inter-medullary stem is almost entirely encapsulated, anyseparating force applied to the porous layer is distributed over theentire interfacial connection and not just to a small portion of thearea mechanically attached to the underlying surface.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

It has been found that polyolefins of the type described in theaforementioned patent application are biocompatible, provide anexcellent environment for tissue ingrowth and are able to withstandstress to which prosthetic devices are normally subjected.

For the porous outer layer polyolefins with a melt index (ASTM D1238)ranging from less than 1 to 5 have been found to work effectively.

The substrate must be formed of a material which is chemicallycompatible with the porous outer layer. For example, when porouspolyethylene is used as the outer layer, a non-porous polyethylenesubstrate with a density of about 0.95 g./c.c. can be used. Further, asin the above example when the same specie of polyolefinic polymer isused for both the porous matrix and substrate the melt index of thesubstrate must be greater than that of the porous outer layer. Forexample, when a polyolefin with a melt index ranging from less than 1 to5 is used to form the porous matrix, the substrate should be formed of amaterial which has a melt index ranging from 5-25. To better understandthe relative properties of the matrix and substrate materials the termmelt index can best be described as being inversely proportional to themelt viscosity of a material, e.g. a low melt index indicates a highviscosity. Thus, the matrix material should be more viscous than thesubstrate.

The substrate can also be formed of a different specie of polyolefinicpolymer than that of the porous matrix. Since different species havedifferent melting temperatures, in order for the inventive method towork when different species are used the melt temperature of thesubstrate should be lower than that of the porous matrix. For example,if polypropylene is used to form the porous matrix, polyethylene can beused to form the substrate since it melts at a noticeably lowertemperature than polypropylene.

The underlying structural member which can be formed of stainless steelor vitallium may be provided with surface irregularities, which can bein the form of, for example, annular rings or shallow grooves which forma diamond-shaped pattern (e.g. knurled). Annular rings 3 mm. wide and 1mm. deep have been found to provide good results.

The substrate is applied to the underlying structure in a thin uniformlayer. The thickness can vary from about 0.5 mm. for a finger jointprosthesis to about 1 mm. for a larger hip joint prosthesis. Thesubstrate can be applied, for example, by coating the structural memberby any of the following known methods: (1) dipping the structuralmember, which has been heated to above the melting point of thesubstrate material, into a fluidized bed of powdered material, (2)electrostatic spraying where the structural member, which may or may notbe heated, is electrically charged and powder is sprayed onto the memberfrom a nozzle with the opposite charge, (3) dipping the structuralmember into a liquid substrate material, or (4) forming a mold forproviding a space around the structural member and filling the mold witha suitable powder and externally applying energy by a dielectric meansto a temperature above the melting point of the substrate material.

The porous layer can be applied, for example, by methods (1), (2) or (4)as described above for forming the substrate. The porous layer should beat least 1 mm. thick, preferably ranging from 2-4 mm. depending on thetype of prosthesis to which the layer is being applied. The heat orenergy which is applied should be to a temperature above the meltingpoint of the substrate material but below the melting point of theporous matrix which has a higher melt temperature and/or melt viscosity.The metal underlying structure will operate to transfer heat to thesubstrate material causing the substrate to soften simultaneously withconversion of the powdered material into the porous matrix. Thetemperature is such, and can easily be detemined by one with ordinaryskill in the art, that the substrate material will soften and the porousmatrix will maintain its shape so that the substrate material willeasily flow into the adjacent pores and anchor the two layers together.

In this manner a bond is formed between the substrate and porous layerwhich is significantly greater than heretofore possible when the porouslayer was applied directly to the underlying structure. The compositeformed as described above provides a much stronger bond between theouter layer of porous material and the underlying structural memberbecause the porous layer is bonded along its entire surface, instead ofat small localized sections. Further, since the structural component isalmost entirely encapsulated any separating force is applied over theentire contact surface.

It should be understood that those with ordinary skill in the art willbe able to make modifications and improvements which fall within thescope of the above invention, all of which are contemplated as fallingwithin the intended scope of the appended claims.

What is claimed is:
 1. Method for bonding a layer of porous polymericmaterial to a rigid implantable portion of a prosthetic device,comprising the steps of (a) forming a substrate by coating theimplantable portion with material which is chemically compatible withthe porous polymeric material and which has a melt viscosity lower thanthat of the porous polymeric material, (b) covering the substrate with aporous polymeric material in powder form capable of forming said porouspolymeric layer for forming a composite coating on the implantableportion, and (c) heating the substrate and porous polymeric material toa temperature high enough to cause the substrate material to flow intothe adjacent pores and low enough so that the porous material willmaintain its shape so that the porous layer will be bonded to thesubstrate and the outer pores will be open to accommodate tissueingrowth.
 2. The method of claim 1, wherein the implantable portion isprovided with a knurled surface before step (a) is performed.
 3. Themethod of claim 1, wherein the substrate is formed of a non-porouspolymeric material.
 4. The method of claim 1, wherein polyethylenehaving a melt index ranging from 5-25 is used as the substrate andporous polyethylene with a melt index less than 5 is used to form theporous layer.
 5. The method of claim 1, wherein step (a) includesforming the substrate at least 0.5 mm. thick.
 6. The method of claim 1,wherein step (b) includes forming the porous polymeric material at least1 mm. thick.
 7. The method of claim 1, wherein the step of heatingincludes applying heat by dielectric means.
 8. The method of claim 1,wherein step (b) includes forming a mold around at least a portion ofthe rigid implantable portion coated with the substrate, leaving a spacebetween the mold and substrate, and filling the space with a powderedpolymeric material.
 9. The method of claim 8, wherein steps (a) and (b)include simultaneously converting the powdered material into a porousmatrix and softening the substrate enabling it to flow into the adjacentpores when heat is applied.
 10. Method for bonding a layer of porouspolymeric material to a rigid implantable portion of a prostheticdevice, comprising the steps of (a) forming a substrate by coating theimplantable portion with material which is chemically compatible withthe porous polymeric material and which has a melting temperature lowerthan that of the porous polymeric material, (b) covering the substratewith a porous polymeric material in powder form capable of forming saidporous polymeric layer for forming a composite coating on theimplantable portion, and (c) heating the substrate and porous polymericmaterial to a temperature high enough to cause the substrate material toflow into the adjacent pores and low enough so that the porous materialwill maintain its shape so that the porous layer will be bonded to thesubstrate and the outer pores will be open to accommodate tissueingrowth.
 11. The method of claim 10, wherein porous polypropylene isused as the porous matrix and polyethylene is used as the substrate.